The present invention relates to an optical achromatic lens, and more particularly to an optical achromatic lens which can be used for reduction projection aligners (steppers) in ultraviolet lithography, etc., for instance, excimer laser reduction projection aligners (steppers) of XeCl, KrF, etc.
Heretofore, steppers having ultrahigh-voltage mercury lamps as light sources have been used for photolithography in the production of semiconductors. However, since LSIs have become highly integrated recently, g-line (436 nm) and i-line (365 nm) of the ultrahigh-voltage mercury lamp used in conventional steppers have become insufficient in resolution.
To increase resolution, development has been made to provide steppers using as light source excimer lasers having shorter wavelengths such as those of XeCl, KrF, etc. However, to achieve the resolution desired in these steppers, chromatic aberration should be removed. Presently, to remove the chromatic aberration in excimer lasers, two methods were proposed.
One method is to narrow the half width of laser beams to reduce the chromatic aberration to the permitted range, and another method is to use an achromatic lens in an optical system to correct the chromatic aberration.
In the case of the first method in which the chromatic aberration is suppressed to the permitted range by narrowing the half width of laser beams, elements such as etalons and prisms and methods such as injection locking, etc. are used to reduce the half width of laser beams to 0.003-0.005 nm. However, the reduction of the half width of laser beams poses several problems. For instance, to compensate the decrease in laser output due to element losses, the power of the laser should be increased, and for this purpose, the laser beam-generating apparatus itself should be scaled up. In addition, speckle patterns become likely to appear, and it is difficult to provide a large projection area.
On the other hand, in the case of the second method in which the achromatic lens is used in an optical system to correct the chromatic aberration, there are only limited materials capable of transmitting excimer laser efficiently. The achromatic lens is constituted by two types of optical materials having different dispersion, and there has conventionally been proposed a combination of high-purity silica glass and a calcium fluoride single crystal (fluorite). There has also been proposed an achromatic lens constituted by a combination of a lens made of synthetic silica glass containing oxides of transition elements such as titanium, iron, etc., oxides of rare earth elements such as lanthanum, cerium, europium, etc., and a lens made of synthetic silica glass containing no additives, which utilizes the difference in dispersion power between them to remove the chromatic aberration (Japanese Patent Laid-Open No. 63-6512).
However, when fluorite is used as an optical material for lens, it has various problems as compared with glass. First, since the fluorite has low hardness, it is vulnerable to scratches. Also, since it is a single crystal, it has cleavage which makes it difficult to have a smooth lens surface. As a result, it is not easy to conduct optical lapping on the fluorite. In addition, since the fluorite is slightly soluble in water, it is poor in durability. Further, it has poor mechanical strength. Because of these problems, the fluorite cannot easily be formed into large lenses having diameters of 100 mm or more.
For these reasons, excimer laser steppers having optical systems constituted by achromatic lenses have presently not achieved the stage of practical use.
On the other hand, in the case of the lens made of synthetic silica glass containing oxides of transition elements and rare earth elements, these additives cause ultraviolet absorption, resulting in the reduction of transmittance and the generation of fluorescence. Accordingly, these additives are not suitable for achromatic lenses for ultraviolet rays, and rather should be removed.